1. Field of the Invention
The present invention generally relates to an X-ray image-processing apparatus, and more particularly, to an X-ray image processor for transforming a grayscale of either a mask image or a contrast image of an object under medical examination.
2. Description of the Prior Art
In a conventional radiation image-processing apparatus utilizing X-rays or the like as a radiation source, an integrated image (referred to as a "mask image") of, for example, one or more images taken before a contrast medium is injected into a region of interest in an object under medical examination, and an integrated image (referred to as a "contrast image") of, for example, one or more images taken after the contrast medium is injected into the region of interest, are input to a digital subtracter. The digital subtracter calculates the difference between pixel values, or contrast levels of the mask and contrast images, in units of pixels, to extract a specific region of an image where the contrast medium is present. The resulting image is known as "a subtraction image". However, the object under examination such as a patient is often moved in a complicated manner while at least the two images (i.e., the mask and contrast images) are being acquired. In such a case, a motion artifact, resulting from a positional shift between the mask and contrast images, is superimposed on images of blood vessels and internal organs. This undersirable image superposition renders proper diagnosis impossible.
A typical example of X-ray image-processing techniques for acquiring a plurality of mask and contrast images is described, for instance, in U.S. Pat. No. 4,544,949 issued on Oct. 1, 1985 to Kurihara.
In order to overcome the above-mentioned drawback, attempts have been made to shift the contrast image in order to minimize a total of negative component data of subtraction image data designating the region of interest set on the subtracted image, thereby correcting the positional shift.
An example of such an attempt is disclosed in Japanese unexamined (KOKAI) patent application No. 59-151941 opened on Aug. 30, 1984.
However, a contrast (grayscale) level difference between the mask and contrast images exists, caused by time-depending variations in tube current of an X-ray tube. For this reason, since the total of the negative component data of the subtraction image data is used as a criterion, the positional shift between the mask and contrast images is not often corrected with sufficient accuracy.
As is well known in the art, when the tube voltage of the X-ray tube changes, the X-ray wavelength changes accordingly (i.e., the X-ray characteristics, the X-ray transmissivity), whereas the tube current changes, the X-ray tube's output (power) changes, although its wavelength remains the same. The changes in tube current may usually occur during the period necessary for acquiring a plurality of mask and contrast images (i.e., during a relatively long image acquisition time period. As a result, variations occur in the contrast level or grayscale of the acquired images. In other words, when using such a typical subtraction method for acquiring a desired subtraction image, by way of a contrast-level (grayscale) difference, such variations in contrast level constitute a significant drawback.
The present invention has been made in consideration of the above situation, and has as its object to provide a radiation image-processing apparatus capable of accurately correcting a positional shift between the mask and contrast images, and also to provide an artifact-free subtraction image, whereby accurate medical diagnosis can be performed.